US risk assessments generally assume that bridges may collapse when a 100-year flood — a streamflow with 1 percent probability of being exceeded in any given year, or 63 percent over the course of a century — occurs. This assumption underestimates risk, the paper’s authors find, because it fails to capture the full range of stream flow conditions that can cause bridge collapse.

In their analysis, the researchers considered the full variability of floods that could cause collapse, as opposed to the 100-year approach taken previously. As a result, their findings identified a greater sensitivity to changes in the underlying frequency of flooding. This result appears to support the idea that analyses considering a range of flood scenarios, as opposed to a single 100-year threshold, could be more robust and accurate.

Indeed, of the 35 bridges analyzed, 23 were estimated to have collapsed during a water flow of lesser intensity than a 100-year flood level. The authors note that a primary reason for these lower flow collapses is the fact that most of those collapsed bridges were built before modern design standards. Because most US bridges, along with most US infrastructure, pre-date the modern design standards, the results highlight a more general risk that extreme climate events pose to US infrastructure.

The American Society for Civil Engineering gives US bridges a C+, estimating that $123 billion is needed to clear the maintenance backlog. An additional $140-$250 billion over the 21st century may be required to address the increasing risks posed by climate change, according to past research.

"To balance funding between the backlog and climate adaptation, bridge managers will need robust data on collapse risk," said lead author Madeleine M. Flint, an assistant professor of civil & environmental engineering at Virginia Polytechnic Institute and State University. "Our study is a step in that direction."

Noah Diffenbaugh is a professor at Stanford’s School of Earth, Energy & Environmental Sciences. Coauthors of “Historical Analysis of Hydraulic Bridge Collapses in the Continental United States” also include Sarah L. Billington, professor of civil and environmental engineering; and Oliver Fringer and David Freyberg, both associate professors of civil and environmental engineering at Stanford.

The Stanford Woods Institute for the Environment supported this research through the Environmental Venture Projects seed grant program.